This invention relates to a process for heat treating precipitation hardenable nickel and nickel-iron base alloys, particularly for improving the stress rupture properties of such alloys at temperatures of about 1200F (about 650C.) while providing adequate tensile strength.
Precipitation hardenable nickel and nickel-iron base alloys fulfill a variety of industrial and commercial demands. For instance, some parts are required to fit to close tolerances despite being subjected to repeated temperature cycling to elevated temperatures, i.e., rings and seals manufactured for gas turbine and aircraft jet engines. Certain precipitation hardenable nickel-iron base alloys possess low thermal expansion characteristics and thus may be advantageously used to satisfy such requirements.
Articles made from precipitation hardenable nickel and nickel-iron base alloys may be shaped using a wide variety of practices including machining and hot, warm, and cold forming but are often manufactured by hot working. Typically, in hot working, the workpiece is first forged. The forging is then either heat treated or subjected to additional hot working operations, such as hot fabrication or welding, before being heat treated. An example of hot fabrication is the forming of turbine rings by ring rolling, a technique wherein a billet of an alloy is upset hot, a hole is punched out of the middle to form a torus, and then the hollow billet is hot rolled between internal and external working rolls to the desired shape. Another ring forming hot fabrication method is flash butt welding, wherein a bar of an alloy is bent cold into the form of a ring, and the adjacent ends of the bar are then flash welded, that is, an electric current, sufficient to create a zone of molten metal, is passed across the adjoining surfaces of the two ends while the two ends are forced together with pressure sufficient to expel substantially all of the molten metal out of the weld zone, resulting in essentially a solid state weld.
The finished or partially finished article is usually solution treated and aged. Solution treatment is necessary to relieve undesirable residual stress and strain, and in order to put the article into a state satisfactory for subsequent aging. Aging is necessary to impart the mechanical properties required of the article for service at subambient, ambient, and/or elevated temperatures.
There are known heat treatments for precipitation hardenable nickel and nickel-iron base alloys. For example, U.S. Pat. No. 2,570,194, issued to C.G. Bieber et al. on Oct. 9, 1951, is directed to providing improved high temperature creep properties by including in the alloy controlled amounts of chromium, aluminum, titanium, niobium, and zirconium and subjecting the alloy to a heat treatment which includes an intermediate step between a solution treating step and an aging step. This intermediate step calls for heating at 1500-1800F. (about 815-980C.) for 1/2-24 h.
U.S. Pat. No. 3,048,485, issued to C.G. Bieber on Aug. 7, 1962, relates to an alloy containing 25-50 w/o nickel, 8-25 w/o chromium, 1.5-6 w/o titanium, 2-8 w/o molybdenum, 0.01-0.3 w/o boron, and the balance iron plus optional amounts of manganese, silicon, vanadium, tungsten, and copper. The high temperature properties of the alloy are to be developed by a heat treatment which usually includes solution treatment at 1800-2150F. (about 980-1175C.) followed by one or more aging treatments between 1100-1500F. (about 595-815C.). An intermediate or pre-aging treatment between 1500-1750F. (about 815-955C) is mentioned as optional.
U.S. Pat. No. 3,705,827, issued to D.R. Muzyka et al. on Dec. 12, 1972, relates to nickel-iron base alloys, with or without chromium and cobalt, containing niobium, titanium, and aluminum and a heat treatment therefor for bringing out intragranular gamma prime and gamma double prime phases (Ni.sub.3 (Nb,Ti,Al)). These phases have a lower solvus temperature than eta phase (Ni.sub.3 Ti) and delta phase (Ni.sub.3 Nb) formed at the grain boundaries. The use of an 1800F. (about 980C.) solution treatment followed by stabilization at 1650F (about 900C.) (as specified in U.S. Pat. No. 3,048,485) is described as adversely affecting stress rupture life (Col. 7).
U.S. Pat. No. 3,871,928, issued to D.F. Smith, Jr. et al. on March 18, 1975, relates to the heat treatment of nickel alloys containing at least 2 w/o (Nb +Ti), at least 25 w/o Ni, and up to 60 w/o Fe with %Ni +%Fe .gtoreq.50 w/o in which various optional elements including 8 w/o or more Cr may be present. Heat treatment includes solution treating between 1600-1950F. (about 870-1065C.), controlled cooling down to 11OOF. (about
595C.), and aging between 1100-1625F. (about 595-885C.). A triple stage heat treatment is recited for a nickel-iron-chromium alloy calling for solutioning at 1750F. (about 955C.) or above, slow cooling to below the precipitation hardening range to 11OOF. (about 595C.) followed by cooling in air to room temperature, then reheating to 1450-1625F. (about 790-885C.) for about 1 to 24 h, again cooling to room temperature, then aging between 1275-1425F. (about 690-775C.) for 1 to 24 h, cooling to 1100-1200.degree. F. (about 595-650C) and holding for 5 to 24 h, and then cooling to room temperature.
U.S. Pat. No. 3,898,109, issued to S.W.K. Shaw on Aug. 5, 1975, relates to a four stage heat treatment for nickel-chromium-cobalt base alloys which interposes between solutioning and aging steps two intermediate steps, one at 1775-1890F. (about 970-1030C.) and the other at 1600-1700F. (about 870-925C.). Shaw points out the difficulty encountered in enhancing both stress rupture strength and ductility at an elevated temperature level of about 816C (about 1500F) and that two prior art four-stage treatments for nickel-chromium-cobalt base alloy seriously impair either stress rupture life or ductility at 1500F (about 816C.).
U.S. Pat. Nos. 4,445,943 and 4,445,944, both issued to D.F. Smith, Jr. et al. on May 1, 1984, are directed to similar overaging heat treatments applicable to certain low thermal expansion alloys. These treatments are reported to give overaged structures which contribute to high notch strength at temperatures of about 1000F. (about 540C), but with a reduction in tensile strength and ductility. The alloys differ primarily in that up to 5 w/o cobalt is optional in the '943 patent and 5-25 w/o cobalt is present in the '944 patent. Each calls for overaging by heating in an intermediate temperature range, 1425-1550F. (about 775-845C.) for '943 and 1375-1550 F. (about 745-845C) for '944.
U.S. Pat. No. 4,624,716, issued to Noel et al. on Nov. 25, 1986, describes a method for heat treating a nickel base superalloy, which method is reported to improve stress rupture life by controlling carbide precipitation on grain boundaries while promoting intragranular formation of gamma prime particles and carbides.
U.S. Pat. No. 4,200,459, issued to D.F. Smith, Jr. et al. on Apr. 29, 1980, and U.S. Pat. No. 4,487,743, issued to D.F. Smith et al. on Dec. 11, 1984, both describe low thermal expansion, precipitation hardenable, nickel-iron base alloys and heat treatments therefor. U.S. Pat. No. 4,685,978, issued to Smith et al. on Aug. 11, 1987, also describes a heat treatment for a low thermal expansion, precipitation hardenable, nickel-iron base alloy. All three patents report improvements in stress rupture properties by use of primary heat treatments which all consist of solution treatment followed by multi-step aging treatments. Additionally, the 1459 patent reports an optional intermediate step at 1350-1550F. (about 730-845C.).
A significant problem in the manufacture of articles made from precipitation hardenable nickel and nickel-iron base alloys is that after working and heat treatment by methods known in the art, each of which heat treatments is termed a primary heat treatment in this application, a substantial quantity of articles or workpieces fail to exhibit the required quantum of stress rupture ductility and/or life. A related problem occurs when a test specimen representative of a batch of workpieces is subjected to a primary heat treatment and fails to exhibit the required stress rupture properties. The rest of the batch is then considered to be of unacceptable quality and thus unsuitable for primary heat treatment. In both cases, such articles are considered unsatisfactory and heretofore have only been considered useful as scrap or for reworking.